Photoreceptors are highly specialized ciliated epithelial cells that are easily accessible and undergo cell differentiation postnatally. The long-term goal of our research program is to understand the molecular signaling pathways underlying vertebrate photoreceptor development and cell differentiation, with particular focus on how disruption of these pathways leads to early onset vision loss. The objective of this proposal is to determine the role of the protein PRA1 in rod photoreceptor cell differentiation. During development, precursor photoreceptor cells exit the cell cycle and undergo a transformation to produce highly polarized cells with unique sensory specializations. Although morphological differentiation of photoreceptors is well characterized, less is known about the underlying molecular mechanisms. The rd1 mouse retina, characterized by early-onset, rapid degeneration of rod photoreceptors, was the first identified animal model of retinitis pigmentosa. Interestingly, rd1 rods show developmental defects, including failure to undergo normal synaptogenesis and outer segment differentiation, significantly before degenerative changes appear, presenting a model for investigations of photoreceptor development as well as a congenital neuronal disease. In preliminary studies, we have performed microarray analysis of wild type mouse retina compared to rd1 mutant retina at P2-8, prior to onset of photoreceptor apoptosis. Only two genes were consistently and significantly downregulated in the rd1 mouse at all time points: PDE6b, the mutant gene, and Prenylated rab acceptor 1 (PRA1). Small GTPases in the Rab family are known to be critical for vesicular trafficking involved in maintenance of both the outer segment and the synapse in the mature photoreceptor. PRA1 regulates delivery of Rab GTPases to membranes in eukaryotic cells, although its precise function and mechanism of action are poorly understood. We hypothesize that PRA1 regulation of vesicular trafficking is necessary and sufficient for synaptic and outer segment differentiation in rod photoreceptors. We propose to test our hypothesis with the following specific aims: (1) Determine the expression pattern of PRA1 in the adult and developing wild type and rd1 mouse retinas using Western blot and immunohistochemistry, and (2) determine whether PRA1 is necessary and sufficient for synaptic and outer segment differentiation in rod photoreceptors using RNA based loss-of-function and gain-of-function techniques. The proposed work is innovative in being the first to identify the significance of PRA1 in an animal model of neuronal development and disease. Upon completion of this work, we expect to have determined whether PRA1 expression can restore normal rod photoreceptor cell differentiation in the rd1 mouse retina. These results are ultimately expected to have a significant impact on the field of developmental neurobiology and to provide insights into mechanisms underlying aberrant differentiation, leading to improvement in childhood visual health. PUBLIC HEALTH RELEVANCE: The proposed experiments will elucidate mechanisms involved in development and differentiation of a highly specialized neuronal epithelial cell type: the rod photoreceptor cell. The rd1 mouse model of juvenile retinitis pigmentosa will be used to gain insights into the source of developmental aberrations. Greater understanding of these mechanisms could ultimately lead to targeted therapeutic interventions for juvenile retinitis pigmentosa and improved childhood visual health.